Phosphorus‐Doped Graphene Aerogel as Self‐Supported Electrocatalyst for CO2‐to‐Ethanol Conversion

Abstract Electrochemical reduction of carbon dioxide (CO2) to ethanol is a promising strategy for global warming mitigation and resource utilization. However, due to the intricacy of C─C coupling and multiple proton–electron transfers, CO2‐to‐ethanol conversion remains a great challenge with low activity and selectivity. Herein, it is reported a P‐doped graphene aerogel as a self‐supporting electrocatalyst for CO2 reduction to ethanol. High ethanol Faradaic efficiency (FE) of 48.7% and long stability of 70 h are achieved at −0.8 VRHE. Meanwhile, an outstanding ethanol yield of 14.62 µmol h−1 cm−2 can be obtained, outperforming most reported electrocatalysts. In situ Raman spectra indicate the important role of adsorbed *CO intermediates in CO2‐to‐ethanol conversion. Furthermore, the possible active sites and optimal pathway for ethanol formation are revealed by density functional theory calculations. The graphene zigzag edges with P doping enhance the adsorption of *CO intermediate and increase the coverage of *CO on the catalyst surface, which facilitates the *CO dimerization and boosts the EtOH formation. In addition, the hierarchical pore structure of P‐doped graphene aerogels exposes abundant active sites and facilitates mass/charge transfer. This work provides inventive insight into designing metal‐free catalysts for liquid products from CO2 electroreduction.


S1. Experimental and DFT calculation details.
Materials. Graphite powder and potassium bicarbonate (KHCO 3 ) were purchased from Aladdin Reagent Co., Ltd. Phosphoric acid (H 3 PO 4 ) was obtained from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were reagent grade and used as received without further purification. Carbon paper (HCP 030) and Nafion solution (5 wt%) were acquired from Shanghai Hesen Electric Co., Ltd and Sigma-Aldrich, respectively. Ultra-high purity carbon dioxide (99.999%) and argon (99.999%) were supplied from Nanchang Guoteng Gas. Co., Ltd. Ultrapure Millipore water (18.2 MΩ) was supplied by a UP water purification system. Sample preparation. Graphene oxide (GO) was obtained through chemical exfoliation of graphite powders using the modified Hummer's method. [1] P-doped graphene aerogels (PGAs) were synthesized using the hydrothermal method.
Typically, a certain amount of phosphoric acid was mixed with 30 mL GO aqueous dispersion (2 mg mL -1 ). The mixture was sonicated for 1 h to form a uniform suspension, and then transferred into a 50 mL Teflon-lined stainless-steel autoclave and heated at 180 ℃ for 12 h. After cooling to room temperature, the produced hydrogel was washed with water and ethanol, then freeze-dried. Finally, the product was annealed at 900 ℃ for 1 h under N 2 flow. The sample prepared with 1-, 2-, and 3-mL phosphoric acid loading was denoted as PGA-1, PGA-2, and PGA-3, respectively. Further improving phosphoric acid loading will cause the deformation of aerogel. For comparison, the control sample of GA was prepared without phosphoric acid via the same procedure. 4 Electrochemical measurements. The electrochemical performances were determined using a CHI 660E electrochemical working station with a three-electrode H-cell. The cathodic and anodic compartments were separated by the Nafion ® 117 membrane. An Ag/AgCl electrode and a graphite rod are served as the reference and counter electrode, respectively. The self-supporting PGAs and GA can be cut into the desired size and directly used as the working electrode ( Figure S9). All potentials were measured against the Ag/AgCl reference electrode and converted to the reversible hydrogen electrode (RHE) using the equation of E (V RHE ) = E (V Ag/AgCl ) + 0.21 V + 0.0591 × pH. The electrolysis was conducted in a CO 2 -saturated 0.5 M KHCO 3 solution (pH = 7.2) at ambient temperature and pressure. During electrolysis, CO 2 was continuously bubbled into the cathodic compartment at a rate of 20 sccm. The gas products were measured using on-line gas chromatography (GC, Agilent 7890B). The electrolyte after electrolysis was collected and tested by 1 H nuclear magnetic resonance (NMR, Bruker 600 MHz) using a pre-saturation method to suppress the water peak.
For the flow cell test, an Ag/AgCl electrode and Pt foil were used as the reference and counter electrode, respectively. PGA-2 was ground and loaded on a gas diffusion layer (GDL) as the working electrode and 1.0 M KOH (pH = 14.0) was used as the electrolyte. During the tests, the electrolyte was circulated through the cathode compartment at a rate of 12 mL min -1 , and CO 2 gas with a flow rate of 20 sccm was fed to the cathode GDL.
In-situ Raman test. In-situ Raman spectroscopy was performed using a Confocal LabRam HR800 microscope (Horiba Jobin Yvon). Raman signals were collected based on a self-made electrochemical cell, in which graphite rod and Ag/AgCl electrode were used as the counter and reference electrode, respectively. The as-prepared PGA catalyst was used as the working electrode in 0.5 M KHCO 3 electrolyte with continuous CO 2 flowing at 20 sccm on the backside. A 50× objective lenses and a laser wavelength of 532 nm were applied.
Characterizations. X-ray diffraction (XRD) patterns were obtained on a Bruker D8 Advance X-ray diffractometer with a Cu target (λ = 1.5418 Å). The spectra of X-ray photoelectron spectroscopy (XPS) were analyzed using a Thermo Fisher Scientific Escalab 250Xi system with a monochromatic Al-Kα source. The morphologies and microstructures were characterized by scanning electron microscopy (SEM, Hitachi SU800) and transmission electron microscopy (TEM, FEI Talos F200X). Raman spectra were collected using a LabRam HR800 spectrometer (Horiba Jobin Yvon) with a 532 nm laser source. N 2 and CO 2 adsorption-desorption isotherms were measured on a Micromeritics ASAP 2460 apparatus. Before each adsorption measurement, the sample was degassed at 150 ℃ for 12 h.

Calculation of cathodic energy efficiency (EE).
Where E is the applied potential in the experiment, FE EtOH is the Faradaic efficiency of ethanol, E 0 is 0.09 V RHE for the thermodynamic potential of CO 2 reduction to ethanol.
DFT calculations. DFT calculations were executed by VASP with the GGA-PBE 6 method (generalized gradient approximation with Perdew, Burke, and Ernzerh) functional. [2][3][4] The cutoff energy, energy convergence, and force convergence were set as 500 eV, 1×10 -4 eV, and 0.03 eV/Å, respectively. Meanwhile, the gamma point is utilized in Mohkhorst-Pack (MP) grid. [5] In addition, the DFT-D3 method with Becke-Jonson damping was conducted for all calculations. [6] For all absorbed intermediates of CO 2 RR and HER, the binding energy (BE) can be written as: Wherein, Et otal is the whole energy of intermediates absorbed on the slab, E slab is the energy of the basic slabs, and E ads represents the energy of various intermediates.
The variation of Gibbs free energy (ΔG) of each reaction step refers to the calculated hydrogen electrode [7] and the expression can be described as: Here, the energy difference of each reaction is ΔE; ΔZPE is the zero-point energy and ΔS is the entropy difference at T=298.15 K. ΔZPE and ΔS were obtained with displacement as 0.015 Å for all absorbed intermediates. ΔG U is the contribution of the electrode potential to ΔG. For the symbols in ΔG U formula, n represents the number of transferred electrons in each step and U is the applied electrode potential. ΔG pH is the correction of free energy at given pH. In this study, the environment was slightly alkaline with a pH of 7.2. All the temperature is 298.15K and k B is the Boltzmann 7 constant.
The energy of H 2 O, CO 2, and H 2 is calculated by VASP in a vacuum. The correction of energy is obtained from vaspkit. [8] The temperature is chosen as 298.15 K, the pressure is 0.035 atm for H 2 O (l) and 1 atm for CO 2 and H 2 . The energy of OHis derived from G(OH -) = G(H 2 O) -1/2 G(H 2 ) in pH = 0. ΔZPE and TΔS of absorbed intermediates are acquired from vaspkit with a temperature of 298.15 K. [8] In this work, the reaction mechanism can be described as, * + 2CO 2 (g) + H 2 O + e - *COOH + CO 2 (g) + OH -*COOH + CO 2 (g) + e - *CO + CO 2 (g) + OH -*CO + CO 2 (g) + H 2 O + e - *CHO + CO 2 (g) + OH -

S2. Schematic illustration of the synthesis process.
Figure S1. Schematic illustration of P-doped hydrogel preparation.

Figure S16. (a) SEM and (b) TEM images, (c) EDS mappings, (d) XRD patterns, and
high-resolution XPS spectra of (e) C 1s and (f) P 2p of PGA-2 after duration test. 16 13 C NMR spectra of the catholyte after electrolysis using 13 CO 2 and 12 CO 2 as feeding gas on PGA-2 at -0.8 V.     Figure S28e). The high *CO binding energies indicated that the process of *COOH conversion to EtOH cannot be conducted. Besides, the *CO absorbed on the slab in Figure S26f was shown in the rightmost picture in Figure   S29.

S7. Electrochemical behaviors analysis for all catalysts.
24 Figure S29. Reaction pathway from CO 2 to *COOH to *CO on P 1 @ZZG structure.      Figure S35a). For *CH 2 CHOH, the energy barrier towards *CH 2 CH is 1.37 eV, much higher than that of *CH 2 CH 2 OH (0.45 eV, Figures S33 and   35b). The value is also lower than that of *CH 3 CHOH (0.69 eV). Therefore, the whole reaction prefers processing to EtOH rather than C 2 H 4 .
28 Figure S36. Free energy diagram (left) and corresponding stick-ball models (right) of HER at three different pathways (color code: P, purple; H, white; O, red; C, grey).
In Path 1, water was directly split to *OH and *O on different P atoms with an overall energy barrier of 1.82 eV. In path 2, after adsorbing *COOH onto the single-bond P atom, the overall HER progressed around the neighboring P atom with a lower energy barrier of 1.28 eV that was still higher than that of CO 2 RR (0.86 eV).
Thus, HER was greatly depressed on P 2 @ZZG with the applied voltages around -0.86 V.